Researchers at the University of Pennsylvania and the University of Michigan have built the smallest fully programmable autonomous robots ever created. These microscopic machines can swim through liquid, sense their surroundings, respond on their own, operate for months at a time, and cost about one penny each to produce.

Each robot is barely visible without magnification, measuring roughly 200 by 300 by 50 micrometers. That makes them smaller than a grain of salt. Because they function at the same scale as many living microorganisms, the robots could one day help doctors monitor individual cells or assist engineers in assembling tiny devices used in advanced manufacturing.

Powered entirely by light, the robots contain microscopic computers that allow them to follow programmed paths, detect local temperature changes, and adjust their movement in response.

The work was reported in Science Robotics and Proceedings of the National Academy of Sciences (PNAS). Unlike previous tiny machines, these robots do not rely on wires, magnetic fields, or external controls. This makes them the first truly autonomous and programmable robots at such a small scale.

"We've made autonomous robots 10,000 times smaller," says Marc Miskin, Assistant Professor in Electrical and Systems Engineering at Penn Engineering and the papers' senior author. "That opens up an entirely new scale for programmable robots."

Why shrinking robots has been so difficult

Electronics have steadily become smaller over the past several decades, but robotics has not followed the same trajectory. According to Miskin, independence at sizes below one millimeter has remained an unsolved challenge. "Building robots that operate independently at sizes below one millimeter is incredibly difficult," he says. "The field has essentially been stuck on this problem for 40 years."

At everyday scales, motion is shaped by forces such as gravity and inertia, which depend on an object's volume. At microscopic sizes, however, surface-related forces dominate instead. Drag and viscosity become overwhelming, dramatically changing how movement works. "If you're small enough, pushing on water is like pushing through tar," says Miskin.

Because of this shift in physics, conventional robotic designs fail. Small arms or legs tend to break easily and are extremely difficult to manufacture. "Very tiny legs and arms are easy to break," Miskin explains. "They're also very hard to build."

To overcome these limitations, the researchers developed a completely new way for robots to move that works with the physics of the microscopic world rather than fighting against it.

How microscopic robots swim

Fish and other large swimmers move by pushing water backward, generating forward motion through Newton's Third Law. The tiny robots take a very different approach.